Appliances and other machines that generate noise are usually provided with acoustical insulation to reduce the levels of sound emanating from the machines. The unwanted sound from these machines can be caused both by the mechanical operation of the motor within the machine, such as the compressor motor within an air conditioner, and by the vibration of the machine itself. In a residential dwelling, excessive noise may be generated by dishwashers, clothes washers and clothes dryers, which can be annoying to inhabitants of the dwelling.
Conventional acoustical treatments for machines generally comprises sound transmission barriers and sound absorption layers. One form of acoustical insulation involves enclosing the noise source in an insulation structure. A typical form of acoustical insulation is a layer of mineral fiber insulation, such as fiberglass insulation, wrapped around or positioned around the source of unwanted noise. For example, a fiberglass absorber is usually incorporated in the front door panel of an under-the-counter dishwasher. The blanket of glass fibers absorbs some of the sound energy entering the fiberglass board, thereby resulting in a reduced transmission of unwanted sound from the source of sound in the appliance. Further, it is known that the insertion of a reflecting sound barrier within the acoustical insulation also reduces the sound transmission through the insulation product. Reflecting sound barriers in the past have been made of paper and also of a thin layer of polymeric material, as well as of other materials such as asphalt. U.S. Pat. No. 5,094,318 to Maeda et al. discloses a sound absorption material for automobiles which includes a damping layer, which may act as a sound reflection barrier, a sound absorption layer which absorbs acoustical energy, and a surfacing materiel. The damping layer can be purely asphalt, or an asphalt modified with resins and polymers. The sound absorption layer can be a foamed material or a fibrous material such as a mineral fiber blanket.
Merely applying a layered insulation and sound barrier laminate to noise producing equipment has been found to provide unsatisfactory acoustical performance for many noise sources. The insulation material often has inadequate insulating capability for the level of sound energy under consideration, since many appliances and other noise producing machines have limited space for applying acoustical insulation. Further, insulation materials are not usually designed with specific noise sources in mind, and typical acoustical insulation does not address the particular acoustical or tonal characteristics of the source of the unwanted noise.
It would therefore be desirable for an apparatus that includes a noise generating machine to include an acoustical insulation system that would be suitable for effectively acoustically insulating the machine, and could be easily manufactured and installed. It would further be desirable for the apparatus to include an insulation system that exhibits maximum sound absorption at a peak sound frequency of the machine.